1. Technical Field of the Invention
The present invention relates to a process for the preparation of diacids or mixtures of diacids from aqueous solutions derived from the washing of cyclohexane oxidation products.
2. Description of the Prior Art
It is known to oxidize cyclohexane, essentially into cyclohexanone and cyclohexanol, with gas mixtures containing molecular oxygen, in the presence or absence of a catalyst.
Thus, during the air oxidation of cyclohexane, in the liquid phase, without catalysts, cyclohexanol and cyclohexanone are primarily obtained. It is also known in this type of process to eliminate, before distilling these compounds, at least some of the side products formed during the oxidation reaction. This elimination is usually carried out by washing with water and/or using alkaline aqueous solutions, either at the end of the oxidation reaction or during or between the various phases of the oxidation.
The products contained in these washing waters are especially peroxides, in particular acid peroxides, mainly 6-hydroperoxyhexanoic acid and polymers derived therefrom.
These washing waters may also contain hydroxycaproic acid in free or esterified form, adipic acid, glutaric acid and/or succinic acid, as well as 5-formylvaleric acid.
Various processes for upgrading these washing waters, in particular into adipic acid, have been proposed.
Thus, patent FR-A-2,061,956 describes a process for the preparation of adipic acid by oxidation, using molecular oxygen under pressure, of 6-hydroperoxyhexanoic acid in aqueous solution. Patent FR-A-1,594,895 and its first certificate of addition FR-A-2,054,701 describe a process for the preparation of adipic acid by oxidation with nitric acid, optionally in the presence of nitrogen peroxide, of the 6-hydroperoxyhexanoic acid contained in the aqueous washing solution.
Processes for the hydrogenation of the acids contained in the washing waters into corresponding alcohols have also been described.
Thus, U.S. Pat. No. 3,985,814 describes a process for the hydrogenation of the acids contained in washing waters into corresponding alcohols, in the presence of platinum deposited on carbon black, under a pressure of 300 bar and at 265.degree. C. This process appears to be relatively effective, since the degree of conversion of the acids into alcohols is about 95%, but the hydrogenation conditions are very harsh, which makes the process too expensive for industrial use. Moreover, this process, by definition, converts the acids into alcohols, which thus does not provide the desired upgrading into dicarboxylic acids.
A process based on the same reactions, carried out under similar temperature and pressure conditions but using a catalyst chosen from ruthenium, rhenium, cobalt, nickel and chromium, is described in German patent DE-A-1,957,396.
Lastly, French patent FR-1,585,375 describes a process for the preparation of .epsilon.-hydroxyhexanoic acid by catalytic hydrogenation of 6-hydroperoxyhexanoic acid, which must be extracted and separated from the washing waters beforehand, using an organic solvent.
Although, admittedly, these known processes make it possible to obtain diacids, in particular adipic acid, or their alcohol intermediates which can optionally be oxidized, they nevertheless all have at least one of the following drawbacks:
(i) their industrial implementation is too expensive because of the conditions under which the reactions must be carried out; PA1 (ii) the adipic acid thus obtained is of insufficient purity to be used directly as an intermediate in the synthesis of polyamide fibers, thus necessitating a subsequent excessive purification treatment; PA1 (iii) these processes process only part of the washing waters, not all of them. PA1 (a) at least partially deperoxidizing said washing waters by subjecting them to catalytic hydrogenation at a temperature of between about 0.degree. C. and about 100.degree. C., in the presence of at least one platinum group metal; PA1 (b) oxidizing with nitric acid the products obtained in the washing waters after the deperoxidation step.